As a result of steadily lower applicable pollution limits in recent years, numerous devices and methods have been developed for the aftertreatment of exhaust gases of internal combustion engines. Efficient exhaust gas aftertreatment systems have become available, for instance, using catalytic converter systems which use ammonia-containing reducing agents for the conversion of NOx.
In order to achieve a reduction of Holding attachment components in exhaust gases, reduction catalytic converters have been developed, particularly for Diesel engines. In this respect, the distinction is usually made between so-called SCR systems (in English, selective catalytic reduction) and so-called catalytic converters with hydrocarbon trap. On SCR catalytic converters, the nitrogen oxides react selectively with a reducing agent. The NOx reduction takes place continuously at the temperature operating range of the catalytic converter. Ammonia is the reducing agent having the highest selectivity. In a motor vehicle, for example, ammonia may be obtained from urea, whereas catalytic converters having a hydrocarbon trap, for example, store NOx as nitrate, and are periodically regenerated in so-called exhaust gas-rich phases, using hydrocarbons carried along with the fuel of the internal combustion engine.
A system is known from European Published Patent Application No. 0381236 in which, by means of compressed air, a urea-water solution for the removal of nitrogen oxides is metered into the exhaust gases coming from a Diesel engine. However, in this instance, the urea-water solution must first be converted by an additional chemical process into ammonia, whereby the efficiency of the method is made worse, and undesired by-products cannot be excluded.
It is also known within the framework of SCR technology, that one can use ammonia-containing starting materials, such as ammonium carbonate or even ammonia water. However, it is common to all methods that ammonia is not used directly, on account of the difficulty of handling it.
Further disadvantages that show up in using ammonia-containing starting materials are, for instance, relatively low mass-related reducing performance, and possibly a limited usefulness in wintertime, whereby appropriate additional measures become necessary.
Devices and methods for the catalytic reduction of emissions of internal combustion engines are known from U.S. Pat. No. 6,122,909. Among other things, it is mentioned here that, within the framework of SCR methods, ammonia may be added to the exhaust gases so that it may react with nitrogen oxides to form nitrogen and water when a catalytic converter is switched on. However, it should be stated as fact, in this connection, that ammonia is a poisonous gas, and that the use of ammonia in SCR systems is connected with great expenditure.